Indian Journal of Chemistry Vol. 40A, September 200 I, pp. 947-952

Isopropylation of benzene in the vapour phase over zeolite beta modified by lanthanum, cerium, gallium and zinc

S Gnanapragasam, V Krishnasamy & V Mohan* Department of Chemical Engineering, Anna University, Chennai 600025, Indi a

Received 24 February 2001; revised 30 May 2001

Zeolite beta has been sy nthesized with varying Si/AI ratio. Beta-IS has been modified by La)~. Ce3+. Ga3

+ and Zn2+

and characterized by physico-che mical analysis. Isopropylation of benzene with isopropanol has been done in a fixed bed flow type reactor under different experimental conditions and the products analysed by G.c. The influences of cations, aluminium content, reac tion and calcination temperatures. space velocity, molar ratio of benzene to isopropanol (lPA ) and time on stream (TOS) on the activity and se lectivity to cllmene and di-isopropylbenzene (DIPB) have been investigated and discussed.

The synthesis of zeolite beta was reported t and its structure was elucidated2

• It has been shown that zeolite beta is an intergrown hybrid of two distinct but closely related structures. Zeolite beta is a high silica large pore material possessing three dimensional 12-ring pore system, which is expected to have distinct catalytic properties3

Studies on alkane cracking, isomerization of meta­xylene, methylation of toluene were done over beta zeolite4

.5. It has also been used for transalkylations and disproportionation of C7 and C9 aromatics6

. It is found to be effective for the selective formation of cymenes7

.S

and synthesis of di-isopropyl benzene9. The stability of

this zeolite during cumene disproportionation was investigated by Tasi et al. 10.1 1.

The present study involves the synthesis of zeolite beta with varying SiiAI ratios, modification of H-beta-15 by ion exchange with La3+, Ce3+. Ga3+ and Zn2+, their characterization and catalytic activity for the isopropylation of benzene with isopropanol under different experimental conditions.

Materials and Methods (a) Synthesis of zeolite beta with different SilAl ratios

The synthesis by hydrothermal method was can'ied out using aqueous systems consisting of Na20, A120 3.x Si02. 27.5 TEAOH Y H20. The values of x varied from 10 to 60 and that of y from 400 to 450. Tetraethylammonium hydroxide (TEAOH 40% solution), sodium aluminate (containing 28.4%, Na20, 46.8% A120 3, 24.8% H20), tetraethylorthosilicate (TEOS) and demineralised water were used.

The actual experimental procedure followed for the synthesis of six samples of different SiiAI ratios were exactly the same except that amount of materials of silica and alumina sources were different for different samples. One typical synthesis of Sil Al = 15 is described. TEAOH solution (200 g) and sodium aluminate(4.26 g) were transferred to a 500 mL beaker, and the mixture was stirred using a magnetic stirrer until a clear viscous solution was formed. To this 124.8 g of TEOS in 27.8 g of water was added. The whole mixture was stirred vigorously for an hour and then the temperature was increased to 335 K using a hot plate. Ethanol formed by hydrolysis of TEOS was allowed to evaporate. The gel formed was transferred to a Teflon lined stainless steel autoclave and kept in an air oven for crystallization at 433-453 K for 3-4 days. After this period, the autoclave was cooled in cold water. The solid product recovered by filtration was washed repeatedly with demineralized hot water until the pH of the filtrate was 7.0. Finally, the product was dried at 393 K in an air oven over night.

The as-synthesized sodium form of zeolite beta was kept at 623 K in a muffle furance for 2h. Subsequently the temperature was raised and kept at 753 K for 16 h in a stream of pure dry air to decompose the occluded template molecules.

The Na form of zeolite beta was converted to NH/ form by repeated ion-exchange with 2 M NtLCI solution at 353 K until the Na+ content was reduced to less than 0.05% by wet chemical, AAS and rep analyses. The NH/ form was finally converted to H+­form by calcination at 753 K for 16 h.

948 INDIAN J CHEM, SEC. A, SEPTEMBER 2001

Five more samples of SilAl ratios of 7.5, 13, 22, 26 and 32 were prepared similarly. Sample H-beta 15 was ion exchanged with requisite volume of aqueous solutions of lanthanum chloride, cerium chloride, gallium nitrate and zinc nitrate. (to give about 1.0% cation) .

(b) Characterization of the catalysts (i) X-ray analysis- X-ray analysis (Phillips X-ray

diffractometer PW 1710) was done following the method of Yarlagada et al. 12 and the data are shown in Fig. I (Aj to Ajjj). Figure I (Aj) shows that as­synthesized samples are 100% crystalline while cation exchanged samples (Fig. I, Ajjj) retain 97-99% of their original crystallinity. But the crystallinity of zeolite beta-I5 on calcination at 653, 753, 853,953 and 1053 K are 99, 99,83, 75 and 67% respectively (cf. Fig. 1, Ajj). The corresponding surface area values are 497, 591, 436, 389 and 277m2/g. For all the samples the characteristic peaks are located at 28=7.8° and 22.6°. These values are similar to the reported value for this catalyst l 3

. The effect of thermal treatment of beta-15 between 553 to 1053 (Fig. 1, Ajj) brings about a decrease in the peak intensity at 28=22.6° especially for the samples calcined at higher temperatures. This decrease may be due to decrease in crystallinity by self steaming. Similar change observed for this zeolite was attributed to structural instabilit/4

. It is evident that structural instability of zeolite beta occurs from 853K.

(ii) IR analysis-All prepared samples irrespective of the modifications, give peaks in the region 400-1300cm- 1 and the characteristic peaks for them occurs at 526 cm- I

. The peaks of as-synthesized samples in the regions 3100-2800 cm-I and 1500-1300 cm- I assigned respectively to stretching and bending vibrations of the ethyl group of the template, disappeared after calcination at 753 K for 16 h in air. Observations " 1 h d I' 1315 slim ar to t e present one are reporte ear ter . .

(iii) Thermal analysis- Thermal analysis (Netsch, Model ST A 490) of as-synthesized samples shows exothermic weight loses at 593, 650, 725 and 905 K. The first two were assigned to the removal of TEAOH and TEA + cations respectively, while the third one to oxidative decomposition of occluded organic materials in the zeolite. The fourth at 905 K may be caused by structural modification.

(iv) Texture analysis-This consists of surface area and pore volume measurements using a micropore area analyser (Model ASTP 2000 Micromeritics) at liquid nitrogen temperature after degassing the samples overnight at 623 Kat 10-5 Torr and the data are given in

Si/AI = 7-5 Si/AI = 22

2500

2 e (deg)

Fig. I-X-ray diffraction pattems:(Ai)-as-synthesised zeolites; (Aij)­calcined zeolites; and (Ajjj)- cations exchanged zeolites

Table 1. Smaller values of as-synthesized than H-form are due to blockage of the pores which are removed on calcination, as a result, the values register an increase. On exchange with cations, there is a decrease and the decrease is different for different cations due to partial blockage of the pores by the cations.

GNANAPRAGASAM er al.: ISOPROPYLATION OF BENZENE OVER ZEOLITE BETA 949

(v) TPD of pyridine and ammonia-The acidity of the zeolite was determjned by TPD of pyridine and ammonia. About 30 mg of the sample was taken in an ampule and the temperature was raised slowly to 673 K, the sample was kept at this temperature for Ih in an atmosphere of helium flowing at the rate of 20 mUmin. and cooled to room temperature. Pulses of pyridine/ ammonia were introduced into the catalyst bed until it attained saturation at 298 K. It was flushed with helium for one hour. Temperature was raised at the rate of lOo/min . from 298 to 1073 K. The detection of the desorbed pyridine/ammonia was made with a tungsten­rhenium thermal conductivity detector of Perkin-Elmer gas chromatograph.

The TPD of pyridine and ammonia over H-beta of different Sil AI ratios and cation exchanged H-beta-15 are shown in Figs 2 and 3 respectively. Figures 2 and 3 indicate the presence of wide spectrum of acid sites. The strength of acid sites is charaterized by the temperature of peak maximum of the desorption spectrum. The desorption of ammonia (Fig. 3) starts at lower temperature than pyridine (Fig. 2) showing that part of ammonia is physically adsorbed on sites like silanol groups.

(c) Catalytic reactor The reactor system consisted of a fixed bed, down

flow reactor made up of quartz whose inner diameter and length were 10 nun and 30 cm respectively. The reactor was heated to the requisite temperature with the help of a tubular furnace, controlled by digital temperature controller with indicator. A thermocouple made up of chromel-alumel was kept at the mjddle of the catalyst bed to measure the temperature. About 1 g of catalyst (size 40-60 mesh) was taken in the reactor and it was supported on either side with thin layer of

qualtz wool and the rest of the space was filled with ceramic beads. The ceramic beads above the catalyst bed acts as a preheating zone. The reactant was fed into the reactor by a syringe pump at predetermined flow rates. The bottom of the reactor was connected to a condenser and a collector kept in ice in which the products were condensed and collected.

(d) Analysis of the products The liquid products collected for 15 mjn of each run,

which normally covers 1 h, was discarded and analysis was made only of the products collected after this time. This was done to ensure the attainment of steady state for the reaction over the catalyst and to set right the temperature fluctuations, if any due to the starting of the reaction. At steady state the products obtained were analyzed in a Hewlett-Packard gas chromatograph-5890-A, equipped with FlO using a 2 M stainless steel column packed with 5% bentone-34 and 5% diisodecyl phthalate on chromosorb-120. Nitrogen was used as a carrier gas flowing at the rate of 20 mUmin. Injection port and column were kept at 473 K and 493 K respectively and oven temperature was programmed from 333-403K with a heating rate of 2°/min.

(e) Regeneration of the catalyst After each run, the reactor was flushed with nitrogen

to remove the adsorbed molecules and the catalysts are regenerated at 823 K by passing a current of CO2 free air for 5 h. This procedure was found to give satisfactory results.

Results and Discussion The main product of the isopropylation of benzene

with isopropanol is cumene. Small quantities of di­isopropylbenzene (DIPB) is formed along with trace

Table ·I--Properties of as-synthesised, H-fonns and different cations exchanged zeolites

Zeolites Pro~rties of Zeolites Surface Area,mz/g Unit Cell Volume, AJ AIIAI+Si VN2 (mllg)

Micropore,< I OA ° Mesopre 20-300 AO

H-Bela 7.5 436 4027 0.117 0.32 0.06 H- Beta 13 496 3992 0.072 0.29 0.06 H- Beta 15 591 3958 0.063 0.30 0.04 H- Beta 22 626 3938 0.044 0.28 0.04 H- Bela 26 667 3924 0.038 0.29 0.04 H- Bela 32 693 3900 0.028 0.23 0.03 La-H- Beta 15 489 3968 0.063 0.26 Ce-H- Beta 15 503 4003 0.063 0.26 Ga-H- Beta 15 488 3982 0.063 0.29 Zn-H- Beta 15 463 4018 0.063 0.22 TEA- Beta 306 0.20

9S0 INDIAN J CHEM, SEC. A, SEPTEMBER 200 1

amounts other products (others) which contai ns C7 to CIO aromatics. Under the influence of various parameters investigated, it is observed that the overall conversion of benzene is mostly in the range of IS to 20%.

Influence of La3+, Ce3

+. Ga3+ and Zn2

+

Exchange of beta-IS with cations does not improve the conversion of benzene and the formation of cumene (Table 2). However, the amount of DIPS formed over Zn-H-beta- IS and Ga-H-beta-I S are higher than the amount formed over La-H-beta IS and Ce-H-beta-IS. The lower amount from La and Ce exchanged zeolites is due the occurrence of side reactions. The proportion of In-DIPS over La and Ce exchanged zeolites is lower

50

'" 0

x <!J (J)

S;/AI' 32 C 0 0. (J)

<!J .... .... 0

~ .... <!J 0

Go-t+-BEA"5

50 Zn-H-BEA · 15

473 873 273 673 1073

Temperature (K)

Fig. 2-TPD of pyridine over H-beta zeolites and over H-beta- IS exchanged with diffe rent cat ions.

than over Ga and Zn exchanged zeolites (Table 2) which reflects the strong acid character association with exchange of rare earth cations l6 which catalyses the side reactions.

Influences of Al content (SiIAI ratio) and space velocity (WHSV)

The proportion of cumene decreases and that of DIPS increases with decreasing AI content (Table 3). The increase in the latter is due to decrease in the side reaction which requires strong acidity. Decrease in AI content in the zeolite causes a decrease in the acidity. As a result of decrease in acidity, further conversion of DIPS is diminished, resulting an increase in their concentration in the products.

'" 0 2~

x <!J (J)

C 0

7~

0. (J) <!J ... .... .8 2~

U <!J .-<!J 0

7~

2~

7~

373

S;/AI' 22

S;/AI • 15

Lo -H- BEA . 15 Ga-H- BEA %1 5

f\-C04i-BEA " 5 Zn4i-BE A"5

~73 773 273 .. 73

Temperature (K)

673 873

Fig. 3-TPD of ammonia over H-beta zeoli tes and over H-beta-IS exchanged with different cations.

GNANAPRAGASAM el al.: ISOPROPYLATION OF BENZENE OVER ZEOLITE BETA 951

o Cumene 0. Cumene 1- DIP8 o DIPS (a) 100 14

(>

t ;!-0 10~

?: ?: .;; .;; .; 90

U u ., ., Qj 6 Qj III III

BO 0 2 410 450 490 5~0

Reaction Temperature (K)

o Cumene (> Cumene .. DIPS o DIPB (b)

100 IB

;!- ;!-

?: ,., : ~ 90 12 .~ U ·z ., u ., ~ ~

BO 6

0

5 9 I~

Mole ratio B/IPA

Fig. 4-{a) Influence of reaction temperature on the selectivity to cumene and D1PB (di-isopropylbenzene) over H-beta-15. WHSV = 3.91h, B.lPA=7.7 mole/mole. (b) Influence of BIIPA (benene to isopropanol mol ratio) on the selectivity to cumene and DIPB over H-beta-15 . Temp.=483K; WHSV=3.9h.

o Cumene (> Cumene .. DIPB 0 DIPB (a)

100 5 (>

(>

;!-0 t

>- ~ .;; :~ 90 4 .~

u u ., ., ~ Jl III

BO ~ 600 850 1100

Calcination Temperature (K)

(b)

100 o Cumene (> Cumene .. DIPB o DIPS

7

;!- ;!-

,., ?:

:~ 90 ~ --. 5 U u .. .. Qj

Qj III III

0

8 ~

0 2 4 6

Time on stream (h)

Fig. 5--{a) Influence of calcination temperature on the selectivity to cumene and DIPB over H-beta-15 Temp.=483K; WHSV=3.91h; BIIPA=7.7 mole/mole. (b) Influence of time on stream (TOS) on the selectivity to cumene and DIPB over H-beta-15. Temp.=483K; WHSV=3.91h B/IPA mole/mole.

Table 2--1nfluence of cations exchange of H-beta-15 on the conversion and product distribution Temp.=483K; WHSV = 3.91h; B/fPA = 7.7 mole/mole

Conversion/Products (wt%) Zeolite H-Beta-15 exchanged with No. Exchange Lanthanum Cerium Gallium Zinc

Conversion 19.53 18.59 18.63 17.58 17.17 Cumene 18.34 16.75 16.62 15.68 15.34 D1PB 0.79 0.83 0.71 1.28 1.47 m-D1PB 0.53 0.57 0.49 0.86 0.94 p-D1PB 0.26 0.26 0.22 0.42 0.53 mlp 2.04 2.19 2.23 2.05 1.88 Others 0.37 0.70 0.96 0.44 0.30

Table 3--Influence of Sit AI ratio on the conversion and product distribution over the zeolites Temp.=483 K; WHSV = 3.91h; BIIPA = 7.7 mole/mole

Conversion/Products (wt%) SitAI ratio 7.5 13 15 22 26 32

Conversion 20.4 19.60 19.53 18.81 18.46 18.16 Cumene 19.36 18.52 18.34 17.60 17.18 16.74 DIPB 0.52 0.61 0.79 0.85 0.91 0.97 III-D1PB 0.35 0.41 0.53 0.56 0.59 0.62 p-DIPB 0.17 0.20 0.26 0.29 0.32 0.35 mlp 2.05 2.05 2.04 1.93 1.84 1.77 Others 0.42 0.41 0.37 0.36 0.37 0.39

952 INDIAN J CHEM, SEC. A, SEPTEMBER 2001

Table 4 ---Influence of WHSV over H-beta- 15 on the conversion and product distribution Temp.=483K; BfIPA=7.7 mole/mole

ConversionfProducts (wt%)

2.0 3.9

Conversion 20.70 19.53 Cumene 19.60 18.34 D1PB 0.69 0.79 IIl-DlPB 0.48 0.53 p-DIPB 0.21 0.26 lIl/p 2.28 2.04 Others 0.41 0.37

The effects of WHSV on the formations of cumene and DIPB and on the m/p ratio of DIPB (Table 4) are similar to the effects of SiJ AI ratio. However, the decreases caused by an increase in the flow rates (Table 4), in the formation of cumene and other products are more than the decreases caused by SiiAI ratios. This is because, increase in flow rate decreases the residential period of the reactants over the surface of the catalysts. The decrease is thus quite understandable.

Influence of reaction temperature and reactant mole ratios

Increase of reaction temperature from 423 to 543 K was investigated and the data are given in Fig. 4a. The formation of cumene increases up to 483 K beyond which it decreases. Selectivity to total DIPB at 423 K was 13.3% which decreases to 2.3% at 543 K. Though the amount decreases, the mlp ratio remains at a constant value 2.0 throughout. These observations indicate the occurrence of secondary reactions like deallylation and cracking of the side chain with increasing temperature.

The selectivity to cumene increases with increase in molar ratio up to 7.7 beyond which it remained practically constant (Fig. 4b). The selectivity to DIPB decreases continuously with increasing molar ratio of BIIPA, due to decrease in the concentration of isopropanol in the reactants mixture.

Influence of calcination temperature and time on stream

Increase in calcination temperature of the catalyst from 653 to 753 K increases the formation of cumene. The initial increase may be attributed to an increase in Bronsted acid sites by desorption of water molecules form the catalyst surface, while a decrease of them, above this temperature (Fig. Sa) may be caused by a decrease in crystallinity by self steaming (Fig. I , Aii).

WHSVh-J

5.5 6.5 8.0 9.5 10.5

18.3 1 16.97 15.62 14.62 13.84 17.04 15.71 14.34 12.74 11.04 0.82 0.85 0.87 0.91 0.96 0.55 0.57 0.58 0.60 0.62 0.27 0.28 0.29 0.31 0.34 2.04 2.04 2.00 1.94 1.82 0.31 0.27 0.23 0.19 1.7

Time on stream also increases the formation of cumene up to 4 h. Beyond 4th hour the formation of cumene remains constant while the formation of DIPB decreases from the beginning (Fig. 5b). The results indicate that coke deposition does not narrow down the channel dimensions but it affects the active sites.

Acknowledgement One of the authors Prof. V. Krishnasamy thanks the

All India Council for Technical Education for the award of Emeritus Fellowship.

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